11.5 RECAPITULATION
Foam bubbles can be made via supersaturation of a gas (generally CO 2 )or
by beating in air. Emulsions are generally made by agitation. Emulsion
formation can be considered without involving emulsion stability, but foam
formation and instability cannot fully be separated, since the time scales for
formation and changes due to instability overlap. This is primarily because
foam bubbles tend to be larger than emulsion droplets, e.g., 100 versus 1mm.
Important properties of the product determined by the formation process
are (a) the drop or bubble size distribution, (b) the composition of the
surface layers around drops or bubbles, and (c) in emulsions the type (O–W
or W–O).
Foam. An important variable in foam making is the overrun. It is
the resultant of various processes during beating: bubble formation,
drainage, and bubble coalescence. Bubbles cream rapidly, forming a close-
packed layer from which liquid drains. In this way a polyhedral foam is
generally formed. The bubbles are separated by thin films that end in
Plateau borders; the latter form a continuous network through which liquid
can drain. Drainage causes separation into foam and liquid, and the foam
gets an ever higherjvalue. This gives the foam a certain firmness. Many
aerated foods are bubbly foams, meaning thatjis smaller than about 0.6.
Such systems are made more firm by various means, mostly gelation of the
continuous phase.
Making emulsions is simple, but making small drops is difficult,
because the Laplace pressure, which causes the resistance to deformation
and breakup, increases with decreasing drop (or bubble) diameter. To
obtain a stable system, small drops are generally desired. The prime
occurrence in emulsion formation is thus the breakup of drops into smaller
ones. External stresses are needed to overcome the Laplace pressure.
Different forces act in different regimes. Viscous forces, which act in a
direction parallel to the drop surface, can arise in laminar flow. This can be
shear flow or elongational flow, and the latter is more effective in breaking
up a drop, especially if the drop has a high viscosity. Breakup can also be
caused by inertial forces, which act where local velocity fluctuations cause
pressure fluctuations; these forces act normal to the drop surface.
Fluctuations can arise in turbulent flow, and they increase with the intensity
of agitation, given as the dissipation rate of mechanical energy or power
density (in W?m^3 ). Depending on conditions, i.e., drop size and liquid
viscosity, a turbulent flow can nevertheless be laminar close to a drop,
breaking it by viscous forces.